Familial Hypertrophic Cardiomyopathy (FHC) is an autosomal-dominant disease resulting from mutations in genes encoding cardiac contractile proteins, and an important cause of Sudden Cardiac Death (SCD). Genotype/phenotype correlation studies suggest that the prognostic significance of most mutations is related to the degree of cardiac hypertrophy and fibrosis. An exception are several Troponin T (TnT) mutations, which confer a high risk of SCD even in absence of significant cardiac hypertrophy and fibrosis.In vitro studies suggest that these TnT mutations all increase myofilament Ca 2+ sensitivity. To investigate if an increased Ca 2+ sensitivity itself could be pro-arrhythmic, an animal model without the confounding fibrosis and/or hypertrophy would be ideal. We recently generated such a model by transgenic expression of mutant human TnT (179N), which resulted in increased myofilament Ca 2+ sensitivity and lack of cardiac hypertrophy and fibrosis. Our preliminary experiments on single cells show decreased Ca 2+ transients with slower decay rates, and elevated diastolic (Ca2+)i in response to beta-adrenergic receptor agonists. Isolated perfused hearts show a high incidence of ventricular arrhythmias, particularly at higher (Ca)o, associated with afterdepolarizations and altered ventricular action potential waveform. Thus we hypothesize that increased myofilament Ca 2+ sensitivity changes intracellular Ca 2+ signaling and contributes independently to development of afterdepolarizations and ventricular arrhythmias, leading to sudden cardiac death. The alternate hypothesis is that other factors than myofilament Ca 2+ sensitivity (i.e., alterations of Ca 2 + signaling without changes in myofilament Ca 2+ sensitivity) contribute to arrhythmias related to TnT mutations. To address the alternate hypothesis, we will examine a transgenic model that expresses a TnT mutation (R278C), which does NOT change myofilament Ca 2+ sensitivity. We will use the Tg-179N and Tg-R278C transgenic models to test these hypotheses from the molecular to the organ level, Each SPECIFIC AIM will independently test potential mechanistic links between changes in myofilament Ca 2+ sensitivity, in cellular Ca 2+ signaling, and in whole heart electrophysiology that may lead to ventricular arrhythmias. If validated by this research, the proposed mechanism of arrhythmogenesis may be applicable to other FHC mutations and diseases that increase myofilament Ca 2+ sensitivity.